Photodetectors in which light detection cells with avalanche photodiodes (“APDs”) operating in Geiger mode are arranged in arrays have been studied in order to achieve photodetectors with high sensitivity. These photodetectors count the number of photons that are incident thereto. The APD operating in Geiger mode is a photodiode that emits one current pulse per one photon incident thereto. An array including the light detection cells with the APDs provides a current pulse having a height proportional to the number of light detection cells receiving the photons. Thus, the number of photons incident to each photodetector can be measured from the pulse height.
A problem is known for the arrays of light detection cells including APDs operating in Geiger mode that light rays in a wavelength band from the visible light to the far-infrared light are radiated from a light detection cell to which photons enter, and the radiation rays enter adjacent light detection cells (“optical crosstalk”).
A technique is known to solve this problem, in which trenches are formed between adjacent two light detection cells with APDs from a side opposite to the substrate on which the light detection cells are formed, i.e., from the light detection cell side, and filled with black resist or a metal such as tungsten. This blocks light rays from adjacent light detection cells to prevent the optical crosstalk. However, the preventing of optical crosstalk is insufficient.
The optical crosstalk prevention technique has furthermore the following problems. In the case of using black resist as a light blocking material, during the manufacture, the light detection cells with the APDs are generally subjected to a high-temperature process performed at a temperature of 400° C. or more. Therefore, the black resist, which generally does not bear such high temperatures, cannot be actually used.
A metal such as tungsten is generally filled into the trenches by sputtering or chemical vapor deposition (CVD). However, these methods, which are generally used to form thin films, are not capable of filling the trenches efficiently. Therefore, voids may be caused to reduce the reliability.